1. Field of the Invention
The present invention relates to fixing devices and image forming apparatuses, and specifically to a fixing device using a foam-like fixer that fixes resin-containing particles to a recording medium.
2. Description of the Related Art
Image forming apparatuses such as printers, facsimile machines, and copiers form images including characters and symbols based on image information on recording media such as papers, fabrics, and OHP sheets. Particularly, electrophotographic image forming apparatuses have been widely used in offices because they can form a high-definition image on plain paper at high speed. Such electrophotographic image forming apparatuses generally employ a thermal fixing method that includes heating and melting toner on a recording medium and applying a pressure to the melted toner so as to be fixed to the recording medium. Since this thermal fixing method can provide high fixing speed, high fixing-image quality, etc., this method is preferably used.
However, such electrophotographic image forming apparatuses consume about half or more of their total power when heating toner with the thermal heating method. On the other hand, low-power-consumption (energy saving) fixing devices are recently demanded from the viewpoint of environmental issues. In other words, it is desired to have a fixing method that extremely reduces a heating temperature to fix toner or one that does not require heating toner at all. Particularly, a nonthermal fixing method that does not require heating toner at all when fixing the toner to a recording medium is ideal from the viewpoint of low power consumption.
As the nonthermal fixing method, a wet fixing method is known. The wet fixing method uses an oil-in-water drop type fixing agent in which an organic compound that is capable of dissolving or swelling toner and insoluble or hardly soluble in water is dispersed and mixed with water. The wet fixing method includes spraying or dropping the oil-in-water drop type fixing agent onto unfixed toner placed at a predetermined position at the front surface of an object substance, dissolving and swelling the toner, and drying the object substance.
As an example of the wet fixing method, Patent Document 1 proposes a method for bringing a coating roller where a fixer is coated into contact with a recording medium on which unfixed toner has been placed. With this method, however, if the thickness of a fixer layer on the coating roller is thinner than that of an unfixed toner layer so as to coat the recording medium with a slight amount of the fixer, the unfixed toner is attracted by the surface tension due to the liquid film of the fixer at the front surface of the coating roller at the position where the coating roller is separated from the recording medium, which in turn causes the offset of toner particles to the front surface of the coating roller. As a result, an image on the recording medium is remarkably degraded.
Conversely, if the thickness of the fixer layer on the coating roller is substantially thicker than that of the unfixed toner layer, the surface tension due to the liquid film at the front surface of the coating roller hardly acts on the toner particles directly at the position where the coating roller is separated from the recording medium (because the fixer is abundant), which in turn does not cause the offset of the toner particles to the front surface of the coating roller. However, since a large amount of the fixer is coated on the sheet, the toner particles are flowed due to the excessive fixer. As a result, image quality is degraded and a long drying time is required, which causes a problem in fixing response. In addition, a significant residual liquid feeling (wet feeling obtained when an operator touches the sheet with his/her hand) occurs in the sheet. Moreover, when the fixer contains water, or when a large amount of the fixer is coated on a medium containing cellulose such as a sheet, the medium may significantly curl up, which possibly causes the jam of the sheet when the sheet is conveyed inside the image forming apparatus.
Accordingly, when the fixer is coated using a roller, it is really difficult to achieve both the slight coating of the fixer to the toner layer on the sheet so as to improve fixing response, reduce a residual liquid feeling, and prevent the curling of the sheet, and the prevention of toner offset to the fixing roller.
In order to address this problem, Patent Document 2 proposes a technique for performing the same process using a foam-like fixer to remarkably reduce the shift of toner to obtain an excellent image. This technique pays attention to the fact that the foam-like liquid containing a large amount of foam has an extremely low bulk density. Further, a thick fixer layer on a coating roller is required when coating is performed using a roller serving as a contact coating unit so as not to cause an offset to the roller. This means that the fixer at the front surface of the coating roller should have a certain level of volume when uniform coating is performed so as not to cause the offset of resin particles.
On the other hand, an amount of the fixer on the resin particle layer after coating is preferably small in terms of fixing response and residual liquid feeling. This means that the weight of the fixer is desirably low. In order to achieve both a large volume of the fixer at coating and a low weight of the fixer on a medium after coating, the density of the fixer should be low. Thus, the weight of the fixer to be coated can be substantially reduced even if the volume of the fixer is large at the coating. In other words, if the fixer having a low bulk density (value obtained by dividing the weight of the fixer by the volume thereof) is used, it is possible to achieve both the slight coating of the fixer and the prevention of toner offset to the fixing roller.
Further, Patent Document 3 proposes a method for generating small foams having a low bulk density. The method of Patent Document 3 includes a first foam generation step of blowing and stirring air in liquid and generating foam-like liquid having a foam diameter greater than a desired foam diameter. Further, the method also includes a second foam generation step of applying a shearing force to the foam-like liquid having the foam diameter greater than the desired foam diameter generated in the first foam generation step and generating the foam-like liquid having the desired foam diameter.
However, when the foam-like fixer is used as in Patent Documents 2 and 3, another problem arises. That is, when the foams generated by a foam generation unit are supplied under pressure to the coating roller through a tube, a large pressure is required due to the fluid resistance of the foams. The diameter of the foams is required to be smaller than the film thickness of the foams so that the foams are uniformly coated. However, the fluid resistance of the foams increases as the foam diameter becomes smaller. Further, a bulk density is required to be lowered so as to reduce the weight of the fixer on a medium. However, the foams having a low bulk density and a small foam diameter have extremely high fluid resistance. Therefore, a large pressure is required when the foams are supplied under pressure from the foam generation unit to the coating roller through the tube. As a result, a high-capacity pump is required for generating the large pressure.
The foams generated by the foam generation unit are repeatedly broken with time, and the diameter of the foams becomes gradually larger. When the foams, which have not been fixed for a certain period of time, are supplied under pressure from the foam generation unit to the coating roller through the tube, the foams cannot be used for fixing because they do not have desired properties. Therefore, the foams must be discharged and wasted outside a system. Further, when the foam-like fixer is coated on the entire surface of the coating roller, the foam-like fixer, which is coated on areas outside a recording medium where unfixed toner is placed (i.e., areas other than those corresponding to the width of the medium, parts between the mediums), is not used for fixing. The foams attached to the coating roller after fixing are required to be cleaned up. However, the volume of the foam-like fixer is extremely large (i.e., several tens of times as large as the liquid fixer), and thus the collected foams are required to be turned into liquid form immediately. If a large amount of the unnecessary foam-like fixer not used for the fixing exists, cleaning load extremely increases. To this end, a foam generation unit provided inside a film-thickness-controlling and coating unit generates the foam-like fixer, which in turn makes it possible to reduce a distance for supplying the foams under pressure. As a result, the fluid resistance required for supplying the foams under pressure can be reduced. Moreover, for example, if fixing is not performed for a long period of time to thereby change the properties of the foams, an amount of the foam-like fixer that must be discharged outside the system can be reduced. However, when the film-thickness-controlling and coating unit generates foams having a low bulk density and a small diameter, it requires a step of applying a shearing force to the foams and reducing the diameter of the foams in addition to the foam generation step of blowing air into the fixer. As a result, the configuration of the film-thickness-controlling and coating unit becomes complicated. Further, when the fixer is coated on the coating roller toward the width direction of a medium with the configuration of a slit having high fluid resistance and a manifold having low fluid resistance, some of the foams are broken. As a result, the diameter of the foams and the bulk density are increased.
In order to solve the above problems, it is required to arrange the foam generation unit having a small volume near the film-thickness-controlling and coating unit and coat the coating roller with foams immediately after the foams are generated. Thus, the distance for supplying the foams under pressure can be reduced. As a result, the fluid resistance required for supplying the foams under pressure can be reduced. Moreover, if fixing is not performed for a long period of time to thereby change the properties of the foams, an amount of the foam-like fixer that must be discharged outside the system can be reduced.
FIG. 1 shows the foam generation unit that generates foams having a small particle diameter. FIG. 1A is a perspective view of the foam generation unit, and FIG. 1B is a partially plan transparent view thereof. The foam generation unit 17 shown in FIG. 1A is generally called a micro mixer. In the micro mixer is arranged a primary channel part 17-1 composed of a liquid-fixer channel 17-1a which is shaped like a rectangular groove and in which a liquid fixer flows and an air channel 17-1b which is shaped like a rectangular groove and in which air flows. Also, in the micro mixer is arranged a secondary channel part 17-2 composed of plural channels so as to get across the liquid-fixer channel 17-1a and the air channel 17-1b. The primary and secondary channel parts 17-1 and 17-2 are integrally arranged in the micro mixer. The secondary channel part 17-2 is formed by multi-structure U-shaped partition walls. At the top surface of the channels, a lid member is fixed. Also, at a predetermined position of the lid member, a slit opening 17-3 is bored so as to correspond to the plural channels. At a position opposite to the slit opening 17-3, the coating roller (not shown) is provided. In the foam generation unit 17 thus configured, the liquid fixer and air flow in the liquid-fixer channel 17-1a and the air channel 17-1b shown in FIG. 1A, respectively. The liquid fixer and the air are introduced into the secondary channel part 17-2, and then alternately merged and mixed with each other when flowing in an air-and-liquid mixing part 18 having the slit opening 17-3. In this manner, the foam-like fixer is generated and supplied onto a coating roller (not shown) from the slit opening 17-3.
Generally, the micro mixer serving as the foam generation unit shown in FIG. 1 is used for mixing and reacting two types of liquid with each other, but less used for mixing air and liquid with each other. However, it is possible to mix air and liquid with each other to generate foams using a commercially-available micro mixer (micro mixer SSIMM-SS-Ni25 manufactured by IMM company).
In the nonthermal fixing method using the foam-like fixer, the diameter of foams is required to be smaller than at least a toner pile height (thickness of a toner layer). However, it is difficult to reliably generate foams having a substantially small diameter according to the method using the commercially-available micro mixer shown in FIG. 1. Further, in order to generate fine foams, it is effective to reduce the width of the secondary channel part 17-2 shown in FIG. 1 and narrow intervals between adjacent channels. However, the risk of clogging due to foreign matter increases as the width of the secondary channel part 17-2 reduces.
Patent Document 1: JP-A-2006-078537
Patent Document 2: JP-A-2007-219105
Patent Document 3: JP-A-2008-197188